In a storage that stores data, there is a problem that the power consumption for waiting for data transfer is enormous. A system shown in Patent Document 1 is proposed to solve the problem. Patent Document 1 discloses a function of creating an HDD that stores data with high access frequency and an HDD that stores data with low access frequency, sorting the data to a plurality of types of HDDs according to the access frequency and storing the data, and stopping the spindle motor when the HDD storing the data with low access frequency is not accessed for longer than a predetermined time.
Meanwhile, Patent Document 2 discloses a system that satisfies both response speed and power saving by further subdividing the types of HDDs at the storage location according to the response speed required to read out data and by storing data, in which the access frequency is low but requires high data reading speed, in an HDD, in which the spindle is not stopped.
While the systems described in Patent Document 1 and Patent Document 2 stop the spindle motors on an HDD-by-HDD basis to realize power saving, the power saving function such as stopping the spindle on a disk-array-by-disk-array basis can be realized by using a technique called RAID (Redundant Arrays of Inexpensive Disk) to combine a plurality of disks to constitute a high-capacity disk device (hereinafter referred to as “disk array” in the present invention). The power saving function on a disk-array-by-disk-array basis is disclosed in Patent Document 3, for example.    [Patent Citation 1]    JP Patent Publication (Kokai) No. 2001-93220    [Patent Citation 2]    WO2004/049147    [Patent Citation 3]    JP Patent Publication (Kokai) No. 2005-157710